As is known in the art, there are advantages of imaging using the sub-millimeter (Sub-mm) wave portion of the spectrum compared to microwave and infrared. As is also known, sub-millimeter wave imaging systems typically use Focal Plane Arrays (FPAs) in conjunction with focusing optics. To avoid diffraction limits on image resolution, systems large compared with wavelengths are needed while element spacing should be of order half wavelength. For microwave radiation, resolution can be achieved with very large antennas or with moving sources and detectors that form a Synthetic Aperture Radar (SAR). For Long Wavelength Infrared Radiation (LWIR) imaging systems will be smaller. However, atmospheric absorption can be a problem as can be smoke and clouds.
The Sub-mmWave region is a good compromise. The range frequency of interest often quoted is around 220-1000 GHz, see Cowley, A. M. and Sorensen; Quantitative Comparison of Solid State Microwave Detectors; IEEE MTT-14 no.12 pp 588-602 (1966). There are two basic imaging approaches (1) active—where the object being imaged is illuminated by a power source with reflected energy measured and (2) passive—where black body radiation is used to distinguish small temperature differences in the object being imaged in order to render an image. In passive systems, the signal levels are very low compared to active. Thus, the sensors which are used must add very little noise which could obscure the signal of interest. The Figure of Merit often used for this property is Noise Equivalent Power (NEP), see “Zero bias resonant tunnel Schottky contact diode for wide-band direct detection” by Chahal, P.; Morris, F.; Frazier, G. published in Electron Device Letters, IEEE Volume 26, Issue 12, December 2005 Page(s): 894-896. This detector property, that should be as low as possible, depends on the design of the detector. Important is not only the device used but also the architecture of the circuitry around the detector. Some implementation examples are shown in FIGS. 1A and 1B.
In the implementation shown in FIG. 1A, the diode acts to rectify the received incoming RF energy signal producing an additive DC output proportional to the amplitude of the input signal. The Intermediate Frequency (IF) amplifier is then a DC amplifier which is coupled to the diode by a low pass filter such as that shown in FIG. 2. The forward bias DC voltage (V) is selected to maximize a combination of NEP (Noise Equivalent Power) and dynamic range of the detector. A detected unmodulated signal results in a change in the DC voltage across the diode. Hence the lowest detectable signal measured in this way is determined by the background bias current, I, that establishes a voltage V across the low pass filter. There are various noise sources within the biased diode such as shot noise, thermal noise and 1/f noise. However, at low levels the desired signal competes with a voltage floor equal to V. This is illustrated in FIG. 3. It is noted in the figure that the resultant bias voltage acting as a floor limits the low end of power detectability. If the bias current and the hence the lo bias voltage could be reduced, a lower level signal would be detectable. However, the bias point chosen relates to sensitivity and NEP for higher RF energy levels. The ideal for this detection mode would be a zero bias detector. Such detectors are possible but more complex semiconductor implementations compared with Schottky devices.
It is known in the art that Chopped Detection (CD) significantly enhances detector sensitivity in optical imaging systems. In CD optical detection systems a spinning perforated wheel is placed in front of the optical beam. The spinning wheel periodically interrupts the beam. This transforms the DC level of the detector into a pulse modulated signal and thereby enables the use of a high pass filter which blocks the DC bias voltage from influencing the amplifier. A low pass filter at the output of the IF amplifier averages the pulsed output and is therefore representative of the DC level of the received signal. In short, only the DC level of the received signal is up-converted in frequency from DC to a predetermined IF signal (i.e., the chopping frequency), the IF signal is amplified, and the amplified IF signal is then down-converted in frequency back to DC. Chopping frequencies are selected based upon the nature of the filter which can be implemented given the sampling time and the low frequency noise spectrum of the diode. A typical chopping frequency would be in the KHz range.
It is also known in the art that Phase Sensitive Detection (PSD) has advantages over CD. In PSD, the phase and frequency of the chopped signal is replicated in a Phase Sensitive Detector such as that shown in FIGS. 5A and 5B. Without the ability to implement CD, it would not be possible to further improve system performance by employing Phase Sensitive Detection (PSD).
In accordance with the invention, a system is provided for detecting amplitude of radio frequency energy. The system includes: an antenna for receiving the radio frequency energy; a modulator, responsive to a reference frequency signal, for pulse modulating the received radio frequency energy at the reference frequency; a detector for homodyning the pulse modulated signal to convert such pulse modulated signal to a detector output signal having a low frequency component representative of the amplitude of the received radio frequency energy and a high frequency component representative of the amplitude at the reference frequency signal; and a high pass or pass band filter at the reference frequency fed for the detector output signal for passing the high frequency components and for removing the low frequency component.
In one embodiment, the diode is DC biased and the high pass or band pass filter filters out the DC bias and low frequency noise by passing the signal through a series blocking capacitor.
In one embodiment, the system includes a phase detector fed by the reference frequency signal and the high frequency components of the pulse modulated (i.e., chopped) radio frequency energy passed by the high pass or band pass centered at the reference frequency. This produces an output representative of a phase difference between the reference frequency signal and the amplitude of the high frequency components pulse modulated radio frequency energy.
In one embodiment, the modulator includes a Low Noise Amplifier (LNA) fed by the antenna and having a bias fed by the reference frequency signal. The LNA pulse modulates the received radio frequency energy at the reference frequency.
With such an arrangement, high levels of integration of components enable full Phase Sensitive Detection (PSD) capability. Bias modulation of a Low Noise Amplifier (LNA) eliminates the loss and reduction of Noise Figure introduced by a switch in series with the received signal. The effect of PSD is equivalent to utilizing a band pass filter centered at the reference frequency. The filter has vary narrow bandwidth that is determined by the data integration time at the output of the detector. The low pass or band pass (with bandwidth as small as possible relative to the reference frequency) effectively accumulates the results of repeated sampling, thereby averaging noise to zero. Other considerations relating to implementation would be power dissipation and circuit size. In alternative embodiments, a commonly fed local oscillator driving the chopping (i,e., pulse modulating) and PSD could be associated with several detector circuits.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.